Monolithic microwave integrated circuits (MMICs) are integrated circuits designed to operate in millimeter-wave and microwave frequency ranges (e.g., 1 gigahertz (GHz) to 300 GHz). A gallium nitride (GaN)—based MMIC typically includes an active layer of aluminum gallium nitride/GaN (AlGaN/GaN) disposed on a suitable buffer layer. The buffer layer is, in turn, disposed on a silicon carbide (SiC) substrate. Active devices may be disposed on the AlGaN/GaN layer opposite the SiC substrate.
After MMIC fabrication is completed, the SiC substrate must be thick enough, e.g., greater than approximately 50-75 micrometers (μm), to prevent unacceptable transmission losses in transmission lines coupled to the active devices. At such a thickness, the flexibility of employing different types of materials for the substrate is limited.